Many earthmoving and utility type industrial machines have one or more hydraulic lift cylinders for controlling the elevational position of an implement such as a bucket or blade. It is advantageous in some cleanup operations to allow the implement to float along the surface so that the implement follows the contour of the surface without operator intervention. This is typically accomplished by providing the directional control valve with a float position at which the opposite sides of the hydraulic cylinder and the tank are all interconnected to permit the hydraulic cylinder to extend and retract as the implement follows the surface contour. Since the float position is generally detented, it becomes common practice to use the float position for lowering the implement from its raised position during normal operations.
The current trend in earthmoving machines is to install zero leak poppet valves between the directional control valve and the lift and retract sides of the lift cylinder to essentially prevent extension and retraction of the cylinder without an operator command to do so. Poppet valves have also been used in place of control valves to control the flow of fluid to and from the cylinder. A common type of poppet valve used in this environment has a reactive chamber in direct communication with the respective lift or retract side of the lift cylinder and a control chamber defined at the back side of the poppet and communicating with the reactive chamber through a fixed or variable orifice. The reactive surface defining the reactive chamber has a smaller effective surface than the back side of the poppet such that the poppet can be opened and closed by controlling the pressure in the control chamber. Generally, this is accomplished with an area control valve disposed between the control chamber and the tank. It is recognized that the subject invention is not limited to lift cylinders. It is recognized that the subject invention can be used on other fluid cylinders or motors without departing from the essence of the subject invention.
One of the problems encountered therewith is providing a float function in which fluid can pass from one side of the cylinder and/or the tank through the poppet valve to the other side of the cylinder. For example, a negative pressure is commonly generated in one side of the cylinder due to rapid movement of the implement, such as by lowering of the implement supported by the cylinder. It was discovered that this negative pressure communicates with the control chamber of the poppet valve and tends to draw fluid from the tank through the opened area control valve. The higher, atmospheric pressure in the tank acting on the back side of the popper valve moves the poppet valve to its closed position disrupting the float function.
Thus it would be desirable to provide a float condition for a hydraulic control system having a poppet valve disposed between at Least one side of a fluid actuator and a tank, such as, a double acting hydraulic cylinder to permit the cylinder to freely extend and retract in response to external forces without operator intervention.
The present invention is directed to overcoming one or more of the problems as set forth above.